Many olefinically unsaturated organic monomers, for example styrene (with the impact of impurities such as, by way of example, divinylbenzene traces) and especially dienes having conjugated double bonds, such as 1,3-butadiene or isoprene, are prone to the spontaneous undesirable formation of popcorn polymers, for example during the storage and the transportation of these monomers, their recovery or further processing. These popcorn polymers are usually highly crosslinked, insoluble materials, which form foamy, crusty polymer granules having a cauliflower like structure on the walls of tanks, pipework, apparati and reactors.
Popcorn polymerization can result from the action of a variety of factors on the monomer concerned, for example oxygen, heat and rust as well as popcorn polymer particles already present in the monomer, which catalyze popcorn polymer formation. Specifically, it appears that the presence of one or more initiators e.g. water, oxygen, hydrogen peroxide results in the formation of popcorn polymer “seeds” in the organic material. The seeds themselves then perpetuate polymerization, without further requiring an initiator and/or a crosslinking agent; they serve as sites for further polymerization. As the particular mechanism, it is believed that monomer diffuses through the surface of the growing polymer mass, and is added to the polymer at the center thereof. For this reason, such polymerization is referred to as occurring “from the inside out.” Consequently, there is continued incorporation of monomer into the polymer phase, leading to buildup of the popcorn polymer. This continuous incorporation of monomer, added with the crosslinking, implies high internal mechanical strains. These strains explain why the polymers breaks, producing new popcorn polymer seeds. The result is a hard polymeric foulant, which can cause serious equipment and safety concerns if left unchecked.
A particular problem attendant upon popcorn polymer formation is its extreme resistance to deactivation, once present in a system. Some of the seeds become attached to the processing and handling equipment, and cannot be readily removed by mechanical means; moreover, being insoluble in most common solvents, they are virtually impossible to wash out by use of such solvents. Even after equipment and storage facilities have been cleaned thoroughly, residual particles of popcorn polymer remain, and promote unwanted polymer growth. Trace particles remaining in the equipment will stay active for long periods without the presence of monomer, and serve to initiate polymerization when once again contacted therewith.
Popcorn polymer formation is especially critical in the case of conjugated diene monomers, such as 1,3-butadiene or isoprene. Here, popcorn polymerization may be responsible for pipework and reactors becoming plugged and for tank charges polymerizing wholesale and the tanks concerned bursting as a consequence.
U.S. Pat. No. 5,072,064 A relates to Inhibition of popcorn polymer growth by treatment with a compound including a Group IV element, and at least one hydrogen bonded to the Group IV element. This compound can be admixed with organic material from which popcorn polymer is formed, or added to a system for the recovery, use or storage of such organic material.
US 2001-005248 A1 relates to a process for the inhibition of popcorn polymer growth in unstabilized material which comprises olefinically unsaturated organic compounds and is prone to form popcorn polymer, which comprises adding to said material an effective amount of an aliphatic alcohol of the formula ROH where R is a straight-chain, branched or cyclic C3-C20-alkyl or alkylene group, the alkylene group bearing a second hydroxyl group.
Other patents such as U.S. Pat. No. 6,348,598, U.S. Pat. No. 6,495,065, US 2004-0019165, US 2004-0267078 and US 2005-0004413 have described similar stabilization.
Nevertheless even the addition of a stabilizer is not enough to prevent popcorn polymerization particularly in places where the monomer stays or circulates at low speed. By way of example such places are the manholes, the shell side of heat exchangers, dead legs such as pipe to safety valves and storage facilities. It is necessary to clean in due time said equipment and storage facilities to prevent plugging or destruction thereof.
U.S. Pat. No. 5,734,098 explains that during the recovery of light hydrocarbons in ethylene plants, butadiene plants, isoprene plants and the like, distillation towers and associated equipment like heat exchangers and reboilers are fouled by the thermal and/or oxidative polymerization of reactive olefins like butadiene. By placing thickness-shear mode resonator devices into the vapor space, beneath select trays in the tower, the probes could be used to detect the formation of foulant such as the popcorn polymer. Thickness-shear mode resonators may be placed in the vapor space of towers such as primary fractionators, depropanizers, debutanizers, and butadiene purification columns. The thickness-shear mode resonators would be sensitive to the formation of viscoelastic polymer in the vapor phase which would deposit on the resonators. This device is deemed to measure the thickness of the popcorn polymer. This process doesn't work efficiently mainly because the popcorn polymer has not a regular thickness like the fouling caused by cooling water.
It has been discovered that during the popcorn polymerization, as well as any polymer with internal mechanical strain, there is an acoustic emission which can be detected by an acoustic sensor such as a microphone. Advantage of said process is that the acoustic sensor has not to be in direct contact with the popcorn, it can be attached on the outside of the volume in which the unwanted formation of polymer occurs, e.g. the manhole, the exchanger's shell, the pipes, the distillation columns or a storage facility. Advantage of said early detection is that operators can remove said popcorn polymers before a complete plugging or a destruction of a piece of equipment.
The prior art has already described acoustic emissions to monitor processes in which the purpose is to make a polymerization, which means it is exactly the contrary of the present invention.
DD 241480 describes a vinyl acetate emulsion polymerization process. Said polymerization generates an acoustic emission recorded by means of transducers of the basic frequency 100 kHz to 1 MHz, thus said polymerization can be followed. In said emulsion polymerization the polymer is in the form of particles and the motion of particles in the liquid generates an acoustic emission. This is not the acoustic emission generated by the internal strain and breakage of a polymer.
F. Ferrer, E. Schille, D. Verardo and J. Goudiakas have described the sensitivity of acoustic emission for the detection of stress corrosion cracking during static U-bend tests on a 316 L stainless steel in hot concentrated magnesium chloride media, (Journal of Materials Science, Volume 37, Number 13/July, 2002 Pages 2707-2712, Springer Netherlands).
GB 2038851 describes a method of, and the apparatus for, continuously measuring the polymerisation process of vinyl chloride and other monomers, using acoustic principles. An acoustic transducer located in polymerising medium responds to acoustical energy emission from chemical and physical interactions in the polymerisation medium. The transduced signal is acoustically coupled via a scanning sound wave-guide and the resulting electrical signal is relayed via wires to a spectrum analyser which provides an indication of a particular polymerisation process. The waveguide and sensor are located in the polymerisation medium. A piezoelectric twin generates the signal. In said emulsion polymerization the polymer is in the form of particles and the motion of particles in the liquid generates an acoustic emission. The electric terminals of the piezoceramic twin are connected e.g. via a preamplifier to a selective microvoltmeter or to a synchronous detector of a decrementmeter. Due to the pulsating flow of the polymerization system in the reactor space the scanning sound wave-guide is made to oscillate in damped oscillations of mostly own frequency, the logarithmic decrement of which is in correlation with changes in the viscosity of the polymerizing product. Said damped oscillations have nothing to see with our acoustic sensor which doesn't oscillate in the polymerization medium.
WO 03 051929 provides a method of evaluating a commercial gas-phase fluid bed reactor continuity by measuring at least one system variable, filtering the data to demodulate a time series and calculating a signal, which is indicative of reactor continuity. System variables comprise an acoustic emission, a differential pressure, a bed total weight/volume, a fluidized bulk density, a static voltage and a reactor wall temperature. In said gas phase polymerization the polymer is in the form of particles or powder, it has nothing to see with the acoustic emission generated by the internal strain and breakage of a polymer.